1. Field of the Invention
The present invention relates generally to a process for plastic article shaping. In particular, it relates to recycling of reclaimed and purified process material, especially of extrudant-receiving bath material.
2. Prior Art Statement
In the continuous production of filamentary vinyl polymer compositions by the wet spinning technique, it is well-known, common practice to polymerize a chosen vinyl monomer composition in a concentrated aqueous inorganic salt solution and to spin the resulting vinyl polymer solution into a coagulating bath which contains an aqueous solution of the same inorganic salt as that employed in the polymerization step. In the coagulation bath, the concentration of inorganic salt is maintained at a level which affords precipitation of the vinyl polymer to form a coherent filamentary gel. The vinyl polymer filamentary gel is then washed, and the inorganic salt is recovered as a concentrated solution from both the washing and coagulation baths by concentrating the effluents thereof. Such concentrated aqueous inorganic salt solution is recycled into the polymerization step for the purpose of economy of operation of the process. See, e.g., U.S. Pat. Nos. 2,916,348 and 2,558,730.
However, in the recovery of the inorganic salt solution from the washing and coagulation bath effluents, copper ion and other impurities accumulate, adversely affecting the solution polymerization, in addition to the final filamentary vinyl polymer product.
Over the years a number of processes for purifying and/or activating such and similar recovered effluents prior to the re-utilization thereof have been proposed. Of particular pertinence are the following:
1. U.S. Pat. No. 2,364,407 (Walker) discloses the removal of lead ions from a dilute sulfric acid bath used for cellulose coagulation and regeneration. The lead ions are precipitated as insoluble lead sulfide as a result of the introduction of hydrogen sulfide, and the precipitate of lead sulfide is removed by filtration of the dilute sulfuric acid through a bed of coal.
To introduce sulfide ions into a solution utilized in a vinyl polymerization (in accord with the process of the present invention) is not considered obvious to one of skill in this art in view of the Walker reference, because of the well-known fact that reducing agents such as sulfides (e.g., organic sulfides, esp. mercaptans) function as potent chain terminators in vinyl polmerizations. (That is to say, such compounds do not react with peroxides to yield free radicals.) Accordingly, one of skill in this art would reasonably avoid the utilization of such compounds, rationally considering them to have no utility in the instant vinyl polymerization process. That the sulfides presently employed have any utility at all is consequently unexpected; that their utilization results in a markedly enhanced polymerization activity (as hereinafter explicated) is surprising a fortiori.
2. U.S. Pat. No. 4,029,725 (Hurley, et al.) discloses a process for purifying and activating a recovered aqueous inorganic salt solution prior to the recycling and re-utilization thereof in a process for producing filamentary vinyl polymers. The purification step therein comprises complexing metallic ion impurities and adsorbing the resulting metal complexes along with organic molecular impurities on activated carbon. In order to ensure the maintenance of acceptable polymerization activity, an activator may be introduced into the recovered concentrated aqueous inorganic salt solution prior to recycling and re-utilization thereof. This activator is selected from the group consisting of tartronic acid, dihydroxytartaric acid, ethylenediaminetetraacetic acid, and water soluble aldehydes.
In the Hurley, et al. reference, the purification step employs adsorption, i.e., the taking up of a molecular material (e.g., a dissolved metal ion complex) on the surface of a solid. In the instant process, on the other hand, the mechanism of purification is solid particle entrapment by a filter bed. Consequently, the Hurley, et al. reference is not suggestive of the purification afforded by the present process. Moreover, in the Hurley, et al. reference, enhanced activation is achieved by the introduction of one or more of a group of organic compounds, none of which contains or generates the sulfide ions required in the process of the present invention. 3. U.S. Pat. No. 3,702,880 (Matsushita) discloses a purification procedure for a zinc chloride solution to be recycled to a vinyl polmerization step after a spinning operation has been effected. However, according to Matsushita, such a purification procedure must be a very particular oxidative treatment. See esp. Col. 3, line 1-14 and Col. 6, lines 66-69 of the Matsushita reference. Such teachings lead those of skill in the art away from the substance of the instant invention, which makes no use whatever of any such oxidation principle.